Hmm, why can't they be on at the same time? Any particular reason? If not, just use the power window fuse, it only has power when the truck is on, or better yet, get a foglight switch, run the power to that off of the the power window/defrost take your pick. I have all of my accessories, digital rear view, ac inverter, cb and whatnot wired that way. They are only on when the truck the key is in ACC. It also gives me the ability to turn everything off and it it is OEM. Just a thought. (The foglight switch output would go to power your relay, power to the load would come straight off the battery. I keep all of my stuff in a Radioshack hobby box inside the glovebox. Works great, wiring tips from the foglight how-to made by the bossman, just move it around as needed to fit your setup)
I think this may be your best option, it wouldn't automatically turn off with your headlights though, but I am not sure what difference it makes? Aren't most DRL just a lower voltage provided to the regular HL?